A Study Comparing the Safety and Efficacy of Two Dosing Regimens of ABT-874 to Placebo in Subjects With Moderate to Severe Chronic Plaque Psoriasis

* Subject is \>= 18 years of age * Subject had a clinical diagnosis of psoriasis for at least 6 months, and had moderate to severe plaque psoriasis * Subject is judged to be in generally good health as determined by the principal investigator

* Subject has previous exposure to systemic anti-IL 12 therapy * Subject cannot discontinue systemic therapies and/or topical therapies for the treatment of psoriasis and cannot avoid UVB or PUVA phototherapy * Subject is taking or requires oral or injectible corticosteroids * Subject diagnosed with erythrodermic psoriasis, pustular psoriasis, medication-induced or medication-exacerbated psoriasis or new onset guttate psoriasis * Subject considered by the investigator, for any reason, to be an unsuitable candidate * Female subject who is pregnant or breast-feeding or considering becoming pregnant